If I had to pick a tipping point in modern times it would be Wickard v. Filburn in 1942.
The USSC decided that Federal regulators could tell a farmer how much grain he could grew on his own land for his own use. This decision gave the Federal bureaucracy essentially unlimited powers.
But in all reality I put the end of the first republic of the US ended in 1865 when it was decided by force of arms that membership in union was not voluntary.
The second republic probably ended either with Wilson and the establishment of the Federal Reserve or with FDR and exponential growth of the the Feral government.